1. Field of the Invention
The subject method for treating contaminated liquids is directed to a method for removing various undesirable constituents from a liquid. More specifically, the subject method is one capable of removing from a contaminated liquid even heavy metal dissolved constituents present in high concentrations. In particular applications, the subject method is capable of removing dissolved heavy metal impurities present at concentrations an order of magnitude greater than 1,000 parts per million (ppm). The subject method is thus capable of removing dissolved heavy metal impurities without requiring any intermediate step for such things as pH adjustment of the liquid or any introduction of chemical additives to effect chemical precipitation in the liquid.
The potentially harmful effects to living beings of contaminating impurities in liquids, particularly in surface waters, has long been widely recognized. The types of harmful impurities, or contaminants, are as numerous and diverse as their various sources. Of the many potentially harmful contaminants, dissolved inorganics like metal pollutants, especially heavy metal pollutants, are known to be particularly pervasive, both in terms of their apparent toxicity to surrounding life forms, and in terms of the quantities and concentrations in which they are found, for example, in industrial effluents, at Superfund sites, and in the products of dredging operations. Toxic metal pollutants are not subject to destruction via biological or thermal oxidation, as organic industrial pollutants may be. Dissolved inorganic pollutants, such as lead or cadmium, moreover, are generally without any substantial `assimilative capacity`--not as to the earth, water, atmosphere, nor any living organisms.
Many industrial processes yet generate and release into surrounding soils and aquatic systems great quantities of waste materials that persist as long-term sources of heavy metal pollutants. Particularly alarming is the fact that ionic species predominate in these pollutants, making them readily available for biological uptake. Not only by marine life, but ultimately by humans.
Numerous attempts have heretofore been made to treat liquids laden with metal contaminants. These attempts may generally be classified into three broad categories--those methods wherein a dissolved metal contaminant is precipitated by the introduction of an activating composition, and appropriate adjustment of pH; those methods wherein removal of dissolved species of contaminants is attempted by use of membrane processes, or by adsorption, chelation, ion-exchange, and sequestration employing various materials; and, those methods wherein the removal of non-dissolved solid contaminants suspended within a liquid is attempted using various solid/liquid separation techniques such as sedimentation (with flocculating agents), flotation, straining and filtration, screening, gravity separation (centrifuging, magnetic separation), and the like.
Significant drawbacks exist with each category which severely curtail the practicability of its methods for many applications. Precipitating metals by introducing a lime or carbonate composition into the given liquid, for instance, necessitates active and precise control of the operating pH. Moreover, the precipitated sludge material that results from the process is typically of an extremely fine size (actually being gelatinous in nature for certain cases), possesses poor filterability and dewatering characteristics, and, depending on the metal, exhibits very high residual metal solubility at near-neutral pH levels. The process reaction time typically characterizing precipitation processes is, furthermore, quite slow and necessarily requires the additional steps following precipitate formation of coagulation and flocculation, settling, and sand filtration.
These processes present significant challenges--from the need for preserving the proper conditions to permit sufficient settling, to the need for dewatering or other such subsequent procedure, and to the need for handling the material finally yielded. What typically results from filtration is a residue formed of an environmentally unstable sludge material. Proper disposal of such material, too, poses significant challenges fraught with well-recognized environmental implications.
Methods of dissolved metallic ion removal that rely upon membrane or other processes such as adsorption, chelation, ion-exchange, complexation and sequestration by various materials (mosses, algae, bacteria . . . ), and phytoremediation represent high-cost concentration processes. They are hindered by such things as membrane clogging, `spent adsorption sites,` and material overload failures (due to surges in metal concentrations).
Methods of removing dissolved metallic ions employing materials such as chitin, chitosan, and various derivatives thereof are known, but found to be expensive, slow-acting, and overly limited in uptake capacities. Those methods, therefore, prove efficacious, at best, only in applications where dissolved pollutants having very low metal concentrations--typically on the order of no more than 500 ppm--are to be treated.
An additional drawback in these methods has been the lack of attention to such factors as the quality of the starting material from which the chitin or chitosan material is obtained and the purification processes to which they are subjected. Without adequate control of these factors, an unpredictable, non-reproducible altering of the extracted chitin's structure, for instance, may result, potentially diminishing the extracted material's desirable properties and disturbing the consistency and uniformity of the resulting preparations. Consequently, the native chitin structure (and that of any derivatives) are essentially lost, along with the advantages that might otherwise have been realized. Also, the altered chitin product(s) are often of such variability in make-up and properties as to render them virtually useless in many of those applications requiring reproducibility of results and product reliability, as noted in U.S. Pat. No. 4,958,011 issued to Bade.
There is, therefore, a need for a contaminated liquid treatment method adapted for highly efficient, reliable removal of such contaminants as dissolved heavy metal pollutants present even at high concentrations. The need is for a method that is not only simple, but is one which yields an environmentally stable by-product unencumbered by the problems associated with by-products of liquid treatment methods heretofore known.
2. Prior Art
Methods and systems for treating contaminated liquids, even those incorporating organic shell materials, are known in the art. The prior art known to Applicant includes U.S. Pat. Nos. 4,156,647; 3,635,818; 4,882,066; 5,543,056; 5,010,181; 4,125,708; 4,992,180; 5,393,435; 4,031,025; 4,522,723; 5,453,203; 4,285,819; 4,755,650; 3,533,940; 5,336,415; 5,433,865; 3,890,225; 4,933,076; 4,990,339; 4,186,088; 5,114,595; 3,754,789; 3,937,783; 5,169,682; 4,127,639; 4,958,011; 4,199,496; 4,532,267; 4,958,012; 5,281,338; 5,160,622; 5,236,492; 5,057,141; 4,862,975; 5,762,903; 4,684,529; 4,897,896; 2,040,879; and, 3,537,256; as well as Japanese Patent Documents #83/266,122; #87/462,003; #84/266,396; and, #89/246,087. Such known methods and systems, however, fail to recognize and therefore fail to exploit the adaptability of certain organic shell materials to forming a substantially insoluble, granular by-product by consuming various metal ions found in contaminated liquids. Known methods and systems, therefore, fail to realize the high efficiency of metal pollutant removal from the contamninated liquids realized in accordance with the present invention, much less to do so in a manner that yields substantially insoluble nodules that may, subsequently, be handled simply and conveniently for disposal or other purposes.
U.S. Pat. No. 4,156,647, for instance, is directed to a process and apparatus for removing impurities from liquids wherein particulate shrimp, lobster, or crab shells are employed in a liquid treatment vessel to remove metal impurities "present in small quantities" in water streams passed therethrough. The reference acknowledges the prior art recognition of the ion exchange or sequestering properties which render chitin and chitosan somewhat effective in metal uptake. Accordingly, the process seeks very specifically to exploit the chitin found in or extracted from the shell material "or other source" to sequester the metal impurities of the passing water streams.
The process reflects a failure to recognize any beneficial properties of the biomineralized shell materials in removing such metal impurities from the water streams. Rather, the process relies wholly upon properties attributable to the given shell material's incidental chitin content. Consequently, the process precludes from use broad classes of shell materials, such as bivalve molluscs, for being virtually chitin-free, though they are revealed by the present invention to be highly efficacious for the given purpose. Also, the process prescribes an adjustment of the pH in the water stream entering the treatment vessel, and yields extracted metal material in a form that subsequently requires pyrolysis (controlled heating and decomposition) or some other further processing upon recovery from the water stream. Unlike the subject method, the process does not produce a by-product containing the extracted metal in an insoluble, granular form which would render it conveniently recoverable by simple collection means.
The failure to recognize the highly advantageous biogenic properties of shell material wholly unrelated to the presence or absence of chitin therein limits this prior art process to the self-acknowledged removal of only metal impurities "present in small quantities." Quantitatively, it is clear from the prior art expressly referred to by the reference, that such "small quantities" correspond to concentrations from below 100 ppm to approximately 1,000 ppm. Concentrations above 1,000 ppm were, in fact, considered in the prior art to represent "gross amounts" which necessitated, first, the contaminated liquid's pretreatment using a precipitation process. These concentration levels thought to limit the domain of effective treatment in the prior art contrast rather sharply to the remarkably high levels of impurities removed in accordance with the present invention--levels typically ranging for certain metal impurities initially present, for instance, in concentrations between 30,000 ppm and 60,000 ppm. The contrast is particularly striking given that such treatment capacity is realized without any requisite pre- or post-treatment processes (precipitation, pH adjustment . . . ).